Conventionally, a plasma is generated using a single rare gas in plasma processing for oxidizing, nitriding, or oxynitriding the surface of a workpiece such as a silicon semiconductor, forming an oxide film, a nitride film, an oxynitride film, a polysilicon film, an organic EL film, or the like on the surface of a workpiece, or etching the surface of a workpiece by the use of a plasma. As the rare gas, use is made of a krypton (Kr) gas or a xenon (Xe) gas with a large electron-collision cross-sectional area and a low plasma electron temperature for reducing plasma damage on the workpiece (see, e.g. Patent Document 1).
Patent Document 1 discloses an apparatus using Kr as a plasma excitation gas for forming an oxide film and a nitride film. This apparatus has a structure in which a coaxial waveguide, a radial line slot antenna, and microwave introducing windows are provided at an upper portion of a vacuum container serving as a process chamber in the order named from the outer side, a shower plate is provided inside the vacuum container, and a stage with a heating mechanism for placing a workpiece thereon is disposed under the shower plate. In a plasma processing method, the inside of the vacuum container is evacuated, an Ar gas is introduced from the shower plate, then the Ar gas is switched to a Kr gas and the pressure is set to 133 Pa. Then, a silicon substrate (workpiece) having been subjected to dilute hydrofluoric acid cleaning is introduced into the process chamber and placed on the stage and then the workpiece is heated so as to be maintained at 400° C. A microwave is supplied to the radial slot antenna from the coaxial waveguide, for example, for 1 minute, thereby introducing the microwave into the process chamber through the dielectric plates (the microwave introducing windows and the shower plate). In this manner, by exposing the surface of the silicon substrate to a high-density Kr plasma generated in the process chamber, surface-terminating hydrogen is removed. Then, while maintaining the pressure in the process chamber at about 133 Pa, a Kr/O2 mixed gas at a predetermined partial pressure ratio is introduced from the shower plate, thereby forming a silicon oxide film having a nano-order thickness on the surface of the silicon substrate. Then, after once stopping the supply of the microwave, stopping the introduction of the O2 gas, and purging the inside of the process chamber with Kr, a Kr/NH3 mixed gas is introduced from the shower plate and, while setting the pressure in the process chamber at about 133 Pa, the microwave is again supplied to generate a high-density plasma in the process chamber, thereby forming a silicon nitride film having a nano-order thickness on the surface of the silicon oxide film. Further, the introduction of the microwave power is stopped to finish the plasma excitation when the formation of the silicon nitride film is finished, and then the Ar gas is substituted for the Kr/NH3 mixed gas, thereby finishing the oxidation-nitriding process. In this manner, a semiconductor integrated circuit device is manufactured using the above apparatus.
However, the existing amount of a Kr gas and a Xe gas in the natural world is small and thus they are expensive as compared with an Ar gas used in normal plasma processing, and therefore, it has been difficult to use them in the industry.    Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2002-261091